Apparatus for and method of making blanks for self-tapping screws



.1. J. FARRELL. 2,309,053 APPARATUS FOR AND METHOD OF MAKINGVBLvANKS FORI SELF-*TAPPING SCREWS Jan. 19, 1943.

2 Sheets-Sheet 2 Filed Feb. 254 1941 Inwenbov James J. Farrell bg 'ff 7W/M 'His ttorneg.

Patented Jan. 19, 1943 SATES PT i'i iiiglilh APPARATUS FOR AND METHOD F MAKING BLANKS FOR SELF-TAPPING SCREWS .laines J. Farrell, Scotia, N. Y., assigner to General Electric Company, a corporation of New York 3 Claims.

The present invention relates to self-tapping screws.

The object of my invention is to provide an improved self-tapping screw, animp-roved blank from which the self-tapping screw is made, and an improved method and die for forming the blanks. For a consideration of what I believe novel and my invention, attention is directed to the following1r description and the claims appended thereto.

In the accompanying drawing, Fig. 1 is a fragmentary perspective View oi a double blow header for making self-tapping screw blanks; Fig. 2'

is a sectional view showing the blank positioned in front of the die; Fig. 3 is a sectional View of the die taken on line 3--3 of Fig. 2; Fig. 4 is a sectional view showing the blank at the end of the operating stroke of the upsetting punch; Fig. 5 is a sectional view showing the blank at the end of the operating stroke of the heading punch; Fig. 6 is a perspective view of the completed blank; and Fig. 7 is a side elevation of a self-tapping screw made from the blank.

Referring to Fig. 1 of the drawings, there is shown a double blow header for making blanks from which the self-tapping screws are made. It is one of a number of well-known types of heading machines which may be used for making the blanks. The details of the operating mechanism of the header are not important for an understanding of the present invention and such mechanism is accordingly not illustrated. The header has a stationary frame I carrying a closed type upsetting die 2 and a reciprocating carriage 3 carrying upsetting and heading punches 4 and 5. Adjacent the die is a guide 6 through which is automatically fed the wire I from which the blanks are formed. When the proper amount of wire has been fed through the guide 6, a transfer finger 8 moves upward into engagement with the projecting end of the wire, and the wire is cut by a shearing member 9. The transfer nger 8 and the shearing member 9 move in unison during the cutting stroke, and the severed portion of the wire is held between these parts and moved to the position shown in Fig. 2 in which it is in line with the die 2 and the upsetting punch- 4. At this point the carriage 3 moves forward and the blank is engaged by the upsetting punch 4 and forced into the die. The position of the parts at the end of the forward stroke of the carriage is shown in Fig. 4. The portion of the blank between the die and the upsetting punch has been partially upset to facilitate the forming of a head during the following operation.

The body of the blank has been forced into the die and upset to the shape of the die. The reciprocating carriage 3 now moves away from the blank and is raised to bring the header punch 5 in line with the blank. The carriage now moves forward bringing the header punch 5 against the partially upset end of the blank and forming the same to a head 20. As the-carriage 3 moves away from the head the completed blank is knocked out of the die by a plunger II. The above described operations take place automatically in the described sequence during the operation of the header.

While being forced into the die by the upsetting punch d, the entering end ci the blank irst engages an inwardly extending annular shoulder i2 at the inner end of a cylindrical section I2a of the die which reduces the diameter of the entering end. Upon passing the shoulder I2 the entering end of the blank is still further reduced in diameter by engagement with a conical surface i3 on the die which starts at the inner edge of the shoulder I2 and decreases in diameter therefrom. As shown more clearly in Fig. 3, there are a plurality of longitudinal relatively narrow grooves Irl in the conical surface I3 into which metal is squeezed or upset to form ribs I5 on the entering end of the blank. The width of the ribs I5 is kept to a minimum so as to reduce the amount of metal which must be owed into the grooves I4 during heading and also to provide a wide chip-receiving space between the ribs to keep the chips clear of the threads. Keeping the chips away from the threads reduces the danger of stripping the threads. The width of the ribs is determined by the strength required for tapping when the thread-cutting grooves and projections are formed in the rib-s as shown in Fig. '7 The sides I5 of the grooves are perpendicular to the conical surface I3 so that both the leading and trailing edges of the ribs I5 rise abruptly from the conical tapered end of the blank. From adjacent the shoulder I2 the grooves I4 extend parallel to the axis of the blank to the point I1, after which the grooves taper inward toward the axis of the blank. The ribs I5 accordingly have tapered sections IB at the entering end of the blank. During the above described operations the part of the blank within the die 2 is formed to the shape of the die. The completed blank accordingly has a cylindrical body I9 corresponding to the section Ia of the die, a reduced or tapered entering end 2t with ribs I5 thereon corresponding to the conical section I3 of the die, and a shoulder 2| at the junction of the body I9 with the entering end of the blank corresponding to the shoulder I2 of the die.

The blank may be threaded in a thread-rolling machine and the head lil of the blank slotted in a slotting machine to produce the self -tapping screw illustrated in Fig. '7. During this operation full threads 22 are formed on the body I9 of the screw and partial threads 23 are formed in the ribs I5. Since the major portion of the conical entering end 20 of the blank lies below the roo-t diameter of the thread, no threads are formed in the conical end except adjacent the shoulder 2I. The ribs I5 project from the conical end 20 to a height intermediate the root and crest diameters of the thread. The threads IS formed in the ribs are accordingly incomplete threads, as illustrated. The threads formed in the tapered sections I8 of the ribs are less complet-e than the threads formed beyond the point I7. There is accordingly a gradual increase in the size of the threads from the entering end of the screw to the full threads on the body I9 of the blank. This makes the screw easier to use since the tapping or thread-cutting action of the screw takes place gradually.

Because the threads 23 in the tapping or thread-cutting section of the screw are of less height than the threads 22 on the main body of the screw, the screw has a tight or snug t. There is no tendency for the thread-cutting section to cut a thread of larger diameter than the threads 22. This means that the screw can be repeatedly removed and replaced without destroying the snugness of the fit and Without stripping the threads previously cut. The edges of the ribs I5 which rise abruptly from the conical end 20 form sharp cutting edges for the teeth 23. Chips cut by the teeth 23 from the material in which the screw is being threaded are received in the spaces between the ribs I5 which from one aspect can be considered as longitudinal chip-receiving grooves or depressions. Because there are three ribs I5, the screw is self-centering. The self-centering feature would be present if a greater number of ribs were used. If this feature were unimportant, fewer ribs could be used.

The self-tapping screw and blank are being claimed in a divisional application Serial No. 405,557, led August 5, 1941.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A closed type header die for forming a selftapping screw blank by upsetting, having a cylindrical section for receiving the body of the blank. an inwardly extending shoulder at the inner end of said section, a conical section having its maximum diameter at the inner edge of said shoulder, and longitudinal rib-forming grooves in said conical section.

2. In the manufacture of self-tapping screws the step of making a blank for later use in making by thread rolling a self-tapping screw having a threaded body with an interrupted thread at the entering end said step comprising, upsetting the entering end of the body of the blank in a closed type die having a reduced portion for receiving and compressing the entering end of the blank to a diameter less than the root diameter of the thread in the body, and utilizing longitudinal grooves in said reduced portion to form by squeezing ribs of a height intermediate the root and crest diameters of the thread in the body whereby the interrupted thread can be formed by rolling the thread in said ribs.

3. In the manufacture of self-tapping screws the step of making a blank for later use in making by thread rolling a self-tapping screw having a threaded body with an interrupted thread at the entering end, said step comprising forming the entering end of the blank by a forging operation to a conical shape of less diameter than the root diameter of the thread to be produced in the body and with longitudinal ribs outstanding from said conical portion to a height intermediate the root and crest diameters of said thread whereby the interrupted thread can be produced by rolling the thread in said ribs.

JAMES J. FARRELL. 

